Perhaps human beings on Earth are alone in the universe after all.
New research from scientists at the University of California shows that previous estimates of how many habitable planets exist in outer space were hugely exaggerated, according to The Daily Mail.
The researchers assert that many planets heretofore regarded as habitable are in fact “dead planets” enveloped with toxic gas. Lead scientist Professor Timothy Lyons stated, “Imagine a 'habitable zone for complex life' defined as a safe zone where it would be plausible to support rich ecosystems like we find on Earth today. Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined.”
To investigate, Lyons and his colleagues created a computer model of the atmospheric climate and photochemistry (a field that analyzes how different chemicals behave under visible or ultraviolet light) on a range of planets. The researchers began by looking at predicted levels of carbon dioxide, a gas that's deadly at high levels but is also needed to keep temperatures above freezing (thanks to the greenhouse effect) on planets that orbit far from their host stars.
Utilizing computer models, the scientists found that planets that existed at great distances from a star needed so much carbon dioxide to trap heat on their planet in order to support life that the carbon dioxide would be poisonous. NASA scientist Edward Schwieterman explained, “To sustain liquid water at the outer edge of the habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today. That's far beyond the levels known to be toxic to human and animal life on Earth."
The Daily Mail noted, “The new study concludes that carbon dioxide toxicity alone restricts simple animal life to no more than half of the traditional habitable zone. For humans and other intelligent animals, which are more sensitive, the safe zone shrinks to less than one third of that area.”
LiveScience added that the scientists argue that stars close to our solar system such as Proxima Centauri and TRAPPIST-1 don’t have areas near them that are safe for life because the planets around them have high levels of carbon monoxide, which can be deadly.
The research paper states:
When we compare the predicted atmospheric CO2 abundances for planets at the outer edge of the HZ for FGKM main-sequence stars (or equivalent pH values) to levels of acute lethality in a range of complex organisms (Figure 2), we find that predicted atmospheric CO2 is three to four orders of magnitude greater than the highest values estimated for the last 500 million years on Earth. Furthermore, the predicted CO2 at the OHZ boundary is one to two orders of magnitude greater than the upper limits for the most CO2-tolerant complex organisms known.
HZ stands for habitable zone; OHZ means the outer edge of the habitable zone; the acronym FGKM comes from the spectrum of stars from hottest to coolest created by the Morgan-Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type).